Load limiting seat belt retractors are used to reduce the effect on a vehicle occupant of forces generated during a crash. They are designed to absorb some of the initial impact forces to minimize injury caused to the vehicle occupant by interaction with the safety restraint itself, for example by sudden impact of the vehicle occupant with the seat belt webbing. In a severe crash the effect of the vehicle occupant's forward momentum being stopped by a relatively narrow band of webbing can cause injuries. The initial forces of a crash on the vehicle occupant have been reduced by load limiting seat belt retractors and particularly those incorporating torsion bars in the seat belt retractor. Torsion bars are connected in the force transmission path and twist under the forces of a crash thus allowing a limited payout of seat belt webbing and dissipating some of the initial energy of the crash pulse. However, they do not break and when the initial peak of the crash pulse is passed, then the torsion bar holds fast and the seat belt retractor locks against webbing payout in the normal way, restraining the vehicle occupant against impact with internal parts of the vehicle.
The ideal load profile of a load limiting seat belt retractor is an initial steep rise in the force of the torque with a smooth transition to a generally flat, i.e. constant force, line. In practice the load profile tends to rise too slowly and to exhibit undesirable peaks and troughs of torque, with the forces rising over an undesirably long period instead of reaching a plateau quickly. This is due to the combination of elastic and plastic deformation exhibited by a torsion bar.
One arrangement to improve the load profile is to introduce shear pins between the spool of the seat belt retractor and the torsion bar. This raises the gradient of the initial part of the force curve due to the additional force needed to shear the pins. The force is dependent upon the rigidity of the pins.
However shear pins are extra components and increase production times and costs. They must be manufactured to close tolerances to produce the desired performance.